"As a consumer, I am just trying to safeguard my civil legal rights," Mr Dong said on Friday.

His attempt has already been rebuffed by Beijing courts twice, but the case highlights dissatisfaction with China's one-size-fits-all film censorship regime.

The system has no age-based ratings, which means any film release is officially suitable for children, while faceless officials have wide discretion to ban or cut titles for a wide range of loosely defined moral, social or political reasons.

Censors have in the past taken offence at such scenes as the killing of Chinese soldiers by Tom Cruise in last year's Mission: Impossible III and the portrayal of a Chinese pirate by Hong Kong star Chow Yun-Fat in the opening minutes of Pirates of the Caribbean III.

Critics say the system undermines the local film industry by making it difficult to come up with compelling content for adult audiences. It also fuels China's booming trade in uncut pirated versions of imported titles such as Lust, Caution that can easily be bought on the street.

Film industry figures have long called for the introduction of a clearer ratings system. Chinese Hollywood star Gong Li, whose steamy scenes with Colin Farrell were among 20 minutes sliced from the China release of last year's Miami Vice, this year appealed for an age-based system to prevent children from being able to see violent films.

Sarft has not ruled out such a system, but has shown little enthusiasm.

The official Xinhua news agency this year said the regulator's approach was based on the view that "films not suitable for children are not suitable for adults, either".

Mr Dong is hardly alone in feeling annoyed.

"Foreigners can watch the full version of Lust, Caution but Chinese can't," complained "Little Wang" in a posting on the popular Sina.com website. "Surely Chinese are not really more childish. Or is there a worry that we can't take this kind of stimulation?"

Mr Dong's lawsuit reflects increasing efforts by critics of China's government to challenge it on points of law.

The law student has also signed a letter to the State Council, China's cabinet, from Zhai Xiaobo, a Peking University law lecturer, that calls for the revision of "inconsistent" film censorship regulations.

Mr Zhai said challenging the legal basis of censorship of film sex scenes could be a good way of pushing politically cautious film officials to consider wider reform of the film management ­system.

"You can't just throw eggs at a rock," Mr Zhai said. "Starting the discussion with this kind of topic is an approach that Sarft can accept."